[PATCH] Cleanup: unify checks for catalog modification

Hi list,

I happened to notice that there are no less than 14 places in the code
that check whether a relation is a system catalog and throwing the
error "permission denied: "foo" is a system catalog"

The attached patch factors all of those into a single
ForbidSystemTableMods() function. Is this considered an improvement?
Should I add it to CommitFest?

Regards,
Marti